Pure substances Compounds elements

Enthalpies of reaction can also be calculated from individual enthalpies of formation (or heats of formation), AHf, for the reactants and products. Because the temperature, pressure, and state of the substance will cause these enthalpies to vary, it is common to use a standard state convention. For gases, the standard state is 1 atm pressure. For a substance in an aqueous solution, the standard state is 1 molar concentration. And for a pure substance (compound or element), the standard state is the most stable form at 1 atm pressure and 25°C. A degree symbol to the right of the H indicates a standard state, AH°. The standard enthalpy of formation of a substance (AHf) is the change in enthalpy when 1 mol of the substance is formed from its elements when all substances are in their standard states. These values are then tabulated and can be used in determining A//°rxn. 

Elements and compounds constitute the world of pure substances. An element is a substance that cannot be decomposed by any chemical reaction into simpler substances. Elements are composed of only one type of atom and all atoms of a given type have the same properties. Pure substances cannot be separated into other kinds of matter by any physical process. We are familiar with many pure substances water, iron, mercury, iodine, helium, rust, diamond, table salt, sugar, gypsum, and so forth. Among the pure substances listed above, iron, mercury, iodine, diamond (pure carbon), and helium are elements. We are also familiar with mixtures of pure substances. These include the air that we breathe, milk, molasses, beer, blood, coffee, concrete, egg whites, ice cream, dirt, steel, and so on. 

Thus far, we ve been concerned primarily with pure substances, both elements and compounds. 

Pure substances are elements and compounds. They have only one type of particle in their structure. For example, pure water only contains water molecules, and gold solely gold atoms (Figure 2). 

To this point you ve examined many of the properties of matter. You ve also learned how scientists have organized, classified, and described matter by arranging it into various subcategories of components. But there remains another fundamental level of classification of matter the classification of pure substances as elements or compounds. 

Take a moment to recall what you have learned about the organization of matter, using Figure 3-17 as a guide. You know that matter is classified as pure substances and mixtures. As you learned in the previous section, mixtures can be homogeneous or heterogeneous. You also know that elements are pure substances that cannot be separated into simpler substances. There is yet another classification of pure substances—compounds. A compound is a combination of two or more different elements that are combined chemically. Most of the substances that you are familiar with and, in fact, much of the matter of the universe are compounds. Water, table salt, table sugar, and aspirin are examples of common compounds. 

The common argument that only elements are pure substances and that sodium chloride contains sodium and chlorine should be rejected. This is because one measures constant values for the density or for the melting point of sodium chloride, and will find such constants of other compounds in well-known published tables. Having a defined density, melting and boiling point, the related substance is considered as a pure substance, e.g. water or ethanol. Pure substances are elements and compounds ... 

Schummer outlines the spectrum of work that has made up this flourishing of the philosophy of chemistry. Reductionism has been a central issue (see Chapters 9 and 10, by Robin Hendry and G. K. Vemulapalli, respectively, in this volume). Also central are attempts to develop and adapt established concepts from the philosophy of science for chemistry—among others, naturalism, explanation, professional ethics, and themata in the history of science (see Chapters 5-8, by Otto Ted Benfey, Eric Scerri, Johannes Hunger, and Jeffrey Kovac, respectively). Philosophers have pursued conceptual analyses of key chemical concepts such as element, pure substance, compound, affinity (see Chapter 17, by Nalini Bhushan on natural kinds, and Chapter 18, by Michael Weisberg on pure substance). 

There are different types of pure substances. Elements and compounds are both pure substances. An element is a pure substance that cannot be changed into a simpler form of matter by any chemical reaction. Hydrogen and oxygen, for example, are elements. Alternatively, a compound is a substance resulting from the combination of two or more elements in a definite, reproducible way. The elements hydrogen and oxygen, as noted earlier, may combine to form the compound water, H2O. 

On the basis of observation or information given to you, classify matter into the correct category of each of the following pairs heterogeneous or homogeneous, solution or pure substance, and element or compound. (Section 1.4)... 

Pure substances are either elements or compounds (Figure 1.1), whereas mixtures can be either homogeneous or heterogeneous. 

A compound is a pure substance that contains more than one element. Water is a compound of hydrogen and oxygen. The compounds methane, acetylene, and naphthalene all contain the elements carbon and hydrogen, in different proportions. 

Usually a good deal of experimentation is needed before a substance can be considered to be pure. Even then, much more work and study are needed before one can decide with confidence that a given pure substance is an element or a compound. Consider the substance water. Water is probably the most familiar substance in our environment and all of us recognize it easily. We are familiar with its appearance and feel, its density (weight per unit volume), the way in... 

A pure white substance, on heating, forms a colorless gas and a purple solid. Is the substance an element or a compound ... 

Throughout this book the use of a number of standard analytical samples is recommended in order that practical experience may be gained on substances of known composition. In addition, standard reference materials of environmental samples for trace analysis are used for calibration standards, and pure organic compounds are employed as standard materials for elemental analysis. 

Molecule the smallest part of an element or compound (pure substance). 

So far, we have discussed only pure substances. However, most materials are neither pure elements nor pure compounds, and so they are not substances in the technical sense of the term (Section A) they are mixtures of these simpler substances, with one substance mingled with another. For example, air, hlood, and seawater are mixtures. A medicine, such as a cough syrup, is often a mixture of various ingredients that has been formulated to achieve an overall biological effect. Much the same can be said of a perfume. 

FIGURE G.5 The hierarchy of materials matter consists of either mixtures or substances substances consist of either compounds or elements. Physical techniques are used to separate mixtures into pure substances. Chemical techniques are used to separate compounds into elements. 

A limited number of pure substances are available from NIST, primarily clini-cally-relevant compounds such as cholesterol, urea, uric acid, creatinine, glucose, cortisol, tripalmitin, and bilirubin (NIST SRM website). These compounds are certified for purity (greater than 99 %) and are used as primary calibrants in definitive methods for these clinical analytes (see below). Several additional pure substances are available for specific applications such as microchemistry, i.e. elemental composition (acetanilide, anisic acid, cystine nicotinic acid, o-bromobenzoic acid, p-fluoro-benzoic acid, m-chlorobenzoic acid), polarimetric standards (sucrose and dextrose), acidimetric standard (benzoic acid and boric acid). Only three pure substance NIST RMs are available for environmental contaminants, namely the chlorinated pesticides, lindane, 4,4 -DDT, and 4,4 -DDE. 

F is the number of degrees of freedom, i.e. the number of variables of state such as temperature and pressure that can be varied independently, P is the number of phases and C is the number of components. Components are to be understood as independent, pure substances (elements or compounds), from which the other compounds that eventually occur in the system can be formed. For example ... 

Compounds are pure substances that have a fixed proportion of elements. 

Landolt-Bornstein, Volume 19, Thermodynamic Properties of Inorganic Materials, Subvolume Al, Pure Substances. Part 1 Elements and Compounds from AgBr to Bajfl, P- Franke and D. Neuschtitz Guest eds., 1999, http //www.springerlink.com/link.asp id=KH6W2TXlRQ77. 

Finally, let s back up to the definition of chemistry as a science having to do with matter, as stated above. So what are we talking about here Matter is anything that has mass and occupies space. That includes everything that can be perceived by our human senses and a lot of stuff that cannot be. Matter includes, but is not limited to, the elements and compounds (pure substances) and mixtures of pure substances. 

Thermodynamic modelling of solution phases lies at the core of the CALPHAD method. Only rarely do calculations involve purely stoichiometric compounds. The calculation of a complex system which may have literally 100 different stoichiometric substances usually has a phase such as the gas which is a mixture of many components, and in a complex metallic system with 10 or 11 alloying elements it is not unusual for all of the phases to involve solubility of the various elements. Solution phases will be defined here as any phase in which there is solubility of more than one component and within this chapter are broken down to four types (1) random substitutional, (2) sublattice, (3) ionic and (4) aqueous. Others types of solution phase, such as exist in polymers or complex organic systems, can also be modelled, but these four represent the major types which are currently available in CALPHAD software programmes. 

Throughout this discussion, we have been considering pure substances, i.e. substances composed of a single material, whether element or compound. A compound may be molecular or ionic, or both. A compound is a single chemical substance. To anticipate slightly, sodium chloride is an ionic compound that contains two atomic species, Na" and d . If a sample of sodium chloride is formally manipulated to remove some Cl ions and replace them by Br ions in equivalent number, the resultant material is a mixture. The same is true of a sample containing neutral species such as P4, Sg and CsHg. 

Specialists in nomenclature recognise two different categories of nomenclature. Names that are arbitrary (including the names of the elements, such as sodium and hydrogen) as well as laboratory shorthand names (such as diphos and LithAl) are termed trivial names. This is not a pejorative or dismissive term. Trivial nomenclature contrasts with systematic nomenclature, which is an assembly of rules, themselves arbitrary. The function of specialists in nomenclature is to codify such rules so that everyone can use them to identify pure substances, rather like many of us use an alphabet to represent words. There may be more than one way to name a compound or species, and no one way may be superior to all the others. Names also vary in complexity, depending upon how much information needs to be conveyed. For example, a compositional name conveys less information than a structural (or constitutional) name, because this includes information about the arrangement of atoms in space. 

This list of materials diat bum is quite long, and one must not forget that the list includes not only the pure substances such as the elements and compounds that make up the list, but mixtures of those elements and compounds. Examples of mixtures would include natural gas, which is a mixture of methane (principally), ethane, and a few other compounds, and gasoline, which is a mixture of the first six liquid alkanes (pentane, hexane, heptane, octane, nonane, and decane), plus a few odier compounds. Wood (another mixture), and wood-related products, like paper, are excellent fuels, as are many polymers such as rubber, plastics, wool, silk, and the above-mentioned cellulose, which makes wood and paper the excellent fuels that they are. 

A substance can be classified chemically in many ways. One of the simplest ways to classify a substance is as an element or a compound. An element is a pure substance that cannot be changed into a simpler substance by chemical means. Elements are the building blocks of nature all matter is composed of elements. The periodic table is a concise map that organizes chemical elements into columns (groups) and rows (periods) based on their chemical properties. Currently, there are 118 known chemical elements, with whole numbers 1 to 118. These numbers are referred to as the element s atomic number and give the number of protons in the nucleus of an atom of the element. For example, carbon s atomic number is 6 and each carbon atom has 6 protons in its nucleus. The first 92 elements occur naturally, and those above atomic number 92 are synthesized through nuclear reactions using particle accelerators. Element 118 was just confirmed in the fall of 2006, and by now, more elements may have been produced. 

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